The present invention relates to a method for producing a magnetic recording medium and, more particularly, to a magnetic recording medium producing method for applying a ferromagnetic material onto a continuously running belt-like nonmagnetic support employing evaporation, sputtering or the like to thereby form a magnetic layer on the nonmagnetic support.
Heretofore, a coating-type magnetic recording medium, produced by applying onto a desired type of a support various types of ferromagnetic metal powder dispersed in a binder by use of an organic solvent, has been commonly used as a magnetic recording medium. On the other hand, a magnetic recording medium having a ferromagnetic metal thin film as a magnetic recording layer, for example, formed using a vapor deposition method such as vacuum evaporation, sputtering, ion plating, or the like, has been more widely used in view of the recent increase in demand for high-density recording.
For example, the aforementioned magnetic recording medium having a thin film of metal as a recording layer is produced by vacuum evaporation or sputtering of ferromagnetic material onto a flexible nonmagnetic support such as polyester film or the like successively at portions which contact cylindrical cans while moving the support guided by guide rollers or cylindrical cans disposed within a vacuum tank.
In general, the tension of the nonmagnetic support during the running thereof is stabilized by adjusting the driving torque between a supply roll and a take-up roll for the nonmagnetic support or by means of a dancer roll disposed in the conveyance system.
The reason why the tension should be stabilized is as follows. Because there is no influence on the running of the nonmagnetic support by air between the nonmagnetic support and the respective cylindrical can in a vacuum or near vacuum, the nonmagnetic support adheres closely to the surface of the cylindrical can. Accordingly, there often arises a considerable difference between tension values of the nonmagnetic support before and after cylindrical can. Consequently, creases or scratches on the support itself, and on the film formed on the support, often arise due to the resulting tensile forces acting on the nonmagnetic support.
Proposals for stabilizing this tension have been disclosed in Japanese Unexamined Patent Publications Nos. 61-278032 and 61-264514. However, theoretical elucidation has not been provided yet. In particular, in the case where a plurality of cylindrical cans are used, not only operations of adjusting the tension corresponding to the respective cylindrical cans must be carried out separately, but also the production equipment must be adjusted whenever the type of nonmagnetic support is changed. In short, many production problems remain. Further, exact tension adjustment has been very difficult because, in general, the accuracy of the tension adjustment depends on the degree of the operator's skill. Further, curling can readily arise in the magnetic recording medium produced by the aforementioned production process. This is the case because cylindrical cans are used or because the magnetic film and the nonmagnetic support are distorted somewhat differently. In practical use, curl balance must be maintained.
It, however, has proven very difficult to obtain good results at all points though avoidance of heat-induced damage and the like, and curl balance may have to be adjusted independently. This is because various set conditions (adjustment conditions) are unavoidably interrelated. When, for example, the amount of tension is increased for the purpose of preventing heat damage, the tension is complexly accumulated in the form of relatively large internal stress both inside the support and inside the thin film formed by the cylindrical can and, consequently, large curls arise easily in the resulting article.
Accordingly, the tension (one of the conveyance conditions) had to be readjusted whenever supports or films were change to others having different physical properties. In particular, in the case where different types of supports or in the case where the films formed on the supports were different in the quality of material and thickness, respective tension conditions had to be readjusted.
Further, the conveyance conditions established for one apparatus could not be applied to other production apparatuses. Accordingly, it was necessary to determine anew the optimum conditions for each different production apparatus. Further, reproducibility of curls was poor. Moreover, as the result of investigations and development carried out by the present inventors, it was found that magnetic films formed when using independent adjustment of the tension as described above were inferior in durability against friction because the quantity of change of the tension increased during the film preparation process to thereby partially change the internal stress of the film and, consequently, the durability and running characteristic of the magnetic recording medium were less than desirable.